It is known to wirelessly charge rechargeable battery-operated hand-held devices such as toothbrushes, mobile communication devices, electrically operated hand-held power tools or the like. For this purpose, an electromagnetic field is used for inductive energy transmission from an induction charging device to a rechargeable battery device of the hand-held device, the induction charging device and the rechargeable battery device each including a coil, which are positionable at a small distance from one another and thus together essentially form a transformer.
If an electrically conductive foreign object enters into the area of the electromagnetic field, eddy currents may form, which heat the foreign object. The foreign object, if it is magnetizable, may also be heated as a result of remagnetization losses or hysteresis losses. The heating may be substantial, so that an operational reliability of the inductive charging system is no longer ensurable. In addition, the foreign object may withdraw energy from the electromagnetic field, so that the energy transmission to the rechargeable battery device is disrupted.
An inductive charging system is known from German Published Patent Application No. 10 2012 205 693 A1, including an induction charging device for wirelessly transmitting energy to a rechargeable battery device, a resonance transformer of the induction charging device generating an alternating electromagnetic field with a charge coil. A determination unit is provided for detecting an object in the area of the charge coil as a function of an electrical parameter at the resonance transformer. In addition, the induction charging device includes a control and regulating unit for changing a quality factor of the resonance transformer, in order to permit an energy transmission in the case of a low quality factor and a detection of the object in the case of a high quality factor.
Furthermore, a method for foreign object detection for an induction charging device is known from unpublished German Published Patent Application No. 10 2013 212 588, in which a resonance frequency and an associated actual quality of an oscillator circuit of the induction charging device are detected and the actual quality is subsequently compared to a setpoint quality as a function of the resonance frequency. Based on a defined setpoint quality range, a decision is then made about the presence of a foreign object.